The analgesic activity of newly isolated and characterized brain peptides (enkephalins) with in vitro opiate-like properties were assessed following microinjections into the periaqueductal gray matter of the rat brain. Synthetic enkephalin (met- and leu-enkephalin) was found to be relatively weak in eliciting analgesia, whereas a partially purified brain extract was quite potent. Met- and leu-enkephalin may only be breakdown products of a more active brain peptide. The opiate antagonist, naloxone, failed to reverse analgesia produced by electrically stimulating the periaqueductal gray matter in rat. Focal brain stimulation does not appear to produce analgesia by releasing enkephalin in brain.